In optical communication systems, an optical signal which is modulated with a data signal is transmitted from an optical transmitter, and is launched into a fiber-optic transmission link. The present invention provides a method and an apparatus for the measurement of the distribution of the reflectivity along an optical transmission line implemented by using the signal processing based on the cross-correlation function between the data signal with which the optical transmitter is modulated and the back-reflected signal returned to the optical transmitter to provide the in-service monitoring of the fiber-optic transmission link.